Use `memcpy` to copy stack.

src/vm.c

@@ -115,9 +115,7 @@ stack_clear(mrb_value *from, size_t count)
 static inline void
 stack_copy(mrb_value *dst, const mrb_value *src, size_t size)
 {
-  while (size-- > 0) {
-    *dst++ = *src++;
-  }
+  memcpy(dst, src, sizeof(mrb_value)*size);
 }
 
 static void